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Some call it the “happening.” Each September thousands of people gather at night behind the Chapman School in Portland, Oregon, to delight in what is perhaps the world’s largest known tornado of birds pouring out of the sky. Wave after wave of Vaux’s swifts roll in like squadrons on a mission, funneling into the school’s tall brick chimney in giant vortexes. Counters have estimated anywhere from 1,700 to 35,000 birds at a time.

At twilight one evening last September, a thousand people came lugging coolers, unfolding picnic blankets, and carrying binoculars. The crowd was so big it spilled into the school’s front lawn (a less-than-ideal vantage point—kind of like watching a concert from the back of the stage). “It’s free. It’s cool. Like going to the movies,” declared Meryl Redisch, Audubon Society of Portland executive director, who this night joined the legions camped out for the group’s Swift Watch. She proclaimed the event a pure celebration of birds, a living, breathing testament to avian joy for young and old, serious birder or complete novice. “It’s a great introduction to a natural phenomenon. It’s so impressive, beautiful, and mysterious.”

Of course, nature’s beauty is almost always balanced with brutality, as it is here on most nights, when a peregrine falcon, one of a pair living on nearby Fremont Bridge, rolls in. The swifts hadn’t even begun their descent when the aerial display overhead took a dramatic turn. Jaws hung open as the crowd watched the peregrine launch its assault. But the swifts, with their rapid flight and amazing agility, outmaneuvered him. “In setting up an attack, a peregrine often makes a wide arc over the neighborhood high up, flying out of view, then reappearing with turbo thrusters fully engaged,” said Steve Engel, adult education program manager for Portland Audubon. Again the masked intruder swooped in; again it came up empty-taloned.

Finally, on its fourth sortie, the lightning-fast raptor, which can reach speeds of more than 200 miles per hour when diving, nabbed its dinner. There was polite—if, in some quarters, guilty—applause. The swifts may be endearing, but the falcons, recovering from near extinction, have to eat, too.

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Vaux’s (rhymes with foxes) swifts are small, dark, and speedy. They are the western counterpart to the better-known chimney swift, and the smallest of the four swift species in North America. (The black swift and the white-throated swift are the other two.) From a distance they’ve been compared to flying cigars with wings. In fact, they are sometimes mistaken for bats because of their appearance at dusk and the way they move, especially when snatching insects out of the air.

Scientists have determined that the birds migrate 4,000-plus miles along the Pacific Flyway in mid-September, from western Canada to Mexico, Central America, and northern Venezuela, making pit stops on the West Coast to rest and refuel. Chapman is one of the last roosting sites in the Portland metro area. “We do not know how many swifts have been coming for years,” said Redisch. “Why don’t they go to other small chimneys? How do they know when to fly? Does something happen? One of them says, ‘It’s time, you know.’ I love the fact that we can’t answer that, and that people can come up with their own answers.”

The birds began their ritual stop here in the late 1980s after losing much of their native roosting habitat in old-growth Douglas fi r forests with large hollow trees, living and dead. Overall the species has proved adaptable, much like the chimney swift, though populations are declining throughout its range.

Chimneys offer the best available substitutes for hollow trees. The communal roosting serves several purposes, among them providing safety from predators and maintaining warmth. The swifts grip the chimney walls with Velcro-like feet. Sadly, though, the species cannot always catch a break in chimneys either; today’s buildings are often built without them.

As interest in the swift spectacle at Chapman soared through social and conventional media and word of mouth, the school grew so fond of its flying guests that it pretty much adopted them, choosing the swift as the school mascot. So each September, even when classroom temperatures fell as low as 50 degrees, the school’s administrators turned off its furnace for the birds’ benefit, and the students donned sweaters and sweatshirts. In 2000 Portland Audubon collaborated with school fundraisers and corporate sponsors to raise $60,000. A year later the school switched from oil to gas and built a new chimney. The old brick chimney now belongs solely to the swifts.

Personifying the civic pride, Scott Bowler, a science education consultant who taught private school in town for 28 years, has been sitting on this same hill on and off for the past 18. He remembers when a couple of dozen people amounted to a big turnout. Searching for something comparable, he gushed about the snow geese, in flocks of tens of thousands, in Malheur National Wildlife Refuge in southeast Oregon, 300-plus miles away. “I just think this is a unique spectacle,” he said. “There’s nothing like it.”

Meanwhile, capitalism and conservation were mixing across the street. Freckle-faced Jack Clevenger, 12, had set up a table in his front yard to sell beverages, $1 cookies and brownies, and the children’s story Swifty’s BigFlight, written by his mother, Lee Jackson. In her delightful little book, Jackson describedthe sky above as “a maze ofblack birds, tiny crescentmoons twisting and turning alldirections in the wind.”

Of course, drawing crowds to witness those crescent moons does have its downsides. For people living across the street, it’s as if the carnival has come to town. There are crowds, traffic congestion, cigarette butts, and litter on their lawns. Audubon is working with the neighborhood, the school, Portland parks, and the city police to improve traffic enforcement and to post park rangers onsite during the roosting season. Their aim is to erase any trace of the visiting hordes.

One item a lot of visitors do bring home with them is a Vaux’s swift brochure on “living with urban wildlife,” published by Portland Audubon. The group’s volunteers—“Swifties”—man a busy table covered with bird paraphernalia and literature while answering questions and repeating some of the advice in the brochure, such as what to do if you find a young swift in your fireplace.

This night, shortly before people prepared to leave, the last swift plunged into the chimney. And as the spectators folded up their blankets and packed their coolers, a symphony of high-pitched and fast chattering overhead fell to a hush. The swifts settled in for the night, snug and safe in the old brick chimney of a school where they finished at the top of their class.

Vaux's Swift

Range: Breeds from southeastern Alaska south to northern California and east to western Montana. This population winters mainly in Mexico and Central America. Closely related forms (regarded as belonging to the same species) are permanent residents in parts of Mexico and Central America, and in northern Venezuela.

Habitat: Feeds on flying insects caught in the air above any habitat but shows a preference for forested areas. North American birds breed in mature or old-growth forest, either coniferous or mixed, where large hollow snags and dead trees serve as nest sites.

Status: Numbers are difficult to census, but specific surveys in Oregon as well as Breeding Bird Survey results elsewhere suggest a significant decline in numbers in North America since 1980. Little information is available about tropical populations.

Threats/Outlook: For the populations in North America, cutting of old-growth forest reduces the availability of nest sites. Roosting sites for migrants are often in old chimneys, and these stopover sites may be lost when chimneys are capped or torn down. Growing public awareness can help to solve the latter problem, but long-term survival will also depend on protecting old-growth forest in the Pacific Northwest.—Kenn Kaufman

This story originally ran in the July-August 2013 issue as "Tornado Watch.”